IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 58, NO. 3, JUNE 2011 639 Towards Optimal Collimator Design for the PEDRO Hybrid Imaging System Chuong V. Nguyen, John E.Gillam, Jeremy M. C. Brown, David V. Martin, Dmitri A. Nikulin, and Matthew R. Dimmock Abstract—The Pixelated Emission Detector for RadiOisotopes (PEDRO) is a hybrid imaging system designed for the measure- ment of single photon emission from small animal models. The proof-of-principle device consists of a Compton-camera situated behind a mechanical collimator and is intended to provide optimal detection characteristics over a broad spectral range, from 30 to 511 keV. An automated routine has been developed for the op- timization of large-area slits in the outer regions of a collimator which has a central region allocated for pinholes. The optimization was tested with a GEANT4 model of the experimental prototype. The data were blurred with the expected position and energy res- olution parameters and a Bayesian interaction ordering algorithm was applied. Images were reconstructed using cone back-projec- tion. The results show that the optimization technique allows the large-area slits to both sample fully and extend the primary field of view (FoV) determined by the pinholes. The slits were found to provide truncation of the back-projected cones of response and also an increase in the success rate of the interaction ordering al- gorithm. These factors resulted in an increase in the contrast and signal-to-noise ratio of the reconstructed image estimates. Of the two configurations tested, the cylindrical geometry outperformed the square geometry, primarily because of a decrease in artifacts. This was due to isotropic modulation of the cone surfaces, that can be achieved with a circular shape. Also, the cylindrical geometry provided increased sampling of the FoV due to more optimal posi- tioning of the slits. The use of the cylindrical collimator and appli- cation of the transmission function in the reconstruction was found to improve the resolution of the system by a factor of 20, as com- pared to the uncollimated Compton camera. Although this system is designed for small animal imaging, the technique can be applied to any application of single photon imaging. Index Terms—Compton scattering enhancement, multiple pin- hole, PEDRO. I. INTRODUCTION S INGLE photon emission imaging devices are typically based on either mechanical [1]–[3] or electronic (Compton) [4] collimation. Mechanical collimators are composed of high Manuscript received November 15, 2010; revised March 04, 2011; accepted March 21, 2011. Date of publication May 05, 2011; date of current version June 15, 2011. This work was supported by the Cooperative Research Center for Biomedical Imaging Development Ltd (CRC-BID), established and supported under the Australian Government’s Cooperative Research Centers Program. C. V. Nguyen and M. R. Dimmock are with the School of Physics, Monash University, Melbourne, VIC 3800, Australia and also with the Monash Node of the CRC for Biomedical Imaging Development, Melbourne, VIC 3800, Australia (e-mail: chuong.nguyen@monash.edu; matthew.dim- mock@monash.edu). J. E. Gillam is with the Instituto de Fisica Corpuscular (IFIC), Universidad de Valencia-CSIC, Valencia, Spain. J. M. C. Brown and D. V. Martin are with the School of Physics, Monash University, Melbourne, VIC 3800, Australia. D. A. Nikulin is with the Monash Node of the CRC for Biomedical Imaging Development, Melbourne, VIC 3800, Australia. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNS.2011.2134869 Z materials that modulate the photon flux incident on the de- tector. This allows a high resolution estimate of the radio-tracer distribution to be obtained at the expense of system sensitivity. Electronic collimation requires no physical modulation of the incidentflux.However,theresolutionoftheimageestimateislim- ited by the detector position and energy resolutions and Doppler broadening. Typically, highly pixelated semiconductor detectors are utilized as they provide superior energy resolution over con- ventional scintillation detectors. Following a Compton scattered event an additional tracking or interaction ordering step is per- formed to determine the first and second interactions that define the cone of response (CoR). The subsequent back-projection of CoRs from many such events yields a high sensitivity, but gener- ally low resolution, estimate of the radioisotope distribution. The Pixelated Emission Detector for RadiOisotopes (PEDRO) [5] is a proof of principle hybrid imaging system being developed to investigate the combination of mechan- ical and electronic (hybrid) collimation [6]–[9]. The intended energy range for operation is from 30 to 511 keV. The optimiza- tion of this hybrid system should yield image estimates with both high resolution and high sensitivity. This will be achieved through reconstruction of both lines of response (LoRs) from well-defined pinholes in the center of the collimator and modu- lated CoRs from large-area apertures in the outer-regions. The aim of such an optimization is to increase the number of pho- tons which impinge on the detector stack without polluting the pinhole projection data. It is expected that the modulated CoRs should complement the pinhole data, extending the field of view (FoV) and improve the iterative reconstructions. In order to achieve this goal, several constraints must be considered in the design of the large-area slits: • The apertures must be able to focus the incident photons at pre-determined regions of the detector stack. • The photons should be directed in a manner which maxi- mizes the probability of a Compton scatter being the pri- mary interaction mechanism. • The overlap between the pinhole FoV and the large-area slit FoV should be maximized in order that the resulting images can be combined and/or quantitatively compared. This paper focuses on collimator optimization for photons with an incident energy . The experimental pro- totype that is currently being tested is introduced in Section II. The 2D-optimization of slit geometries and the extension to 3D are described in Section III. This section also details the opti- mization procedure and the Compton reconstruction algorithm. Quantified results from Monte-Carlo simulations of the experi- mental prototype are presented in Sections IV and V. Finally, the results and directions for future work are provided in Section VI. 0018-9499/$26.00 © 2011 IEEE